A sheet feed assembly has been widely used in various image-forming apparatuses such as an electronic photocopier, a printer, a facsimile receiver or others in the prior art, in which a sheet material, such as a new printing sheet (used, e.g., for a copy), a used or printed paper, or a transparent sheet, is gripped between a rotatable feed roller and a sheet support plate disposed opposite to the outer circumference of the feed roller and elastically biased toward the feed roller and fed at a desired speed in an optional direction.
FIG. 14 illustrates one of the prior art sheet feed devices used in a sheet delivery part of an image-forming apparatus of such a kind, wherein FIG. 14(a) shows a state in which no printing sheet (or transfer medium) is stocked in a paper delivery part and FIG. 14(b) shows a state in which the printing sheet are stocked therein. This sheet feed device is provided with a feed roller (or a sheet feed apparatus) 1, a sheet delivery cassette 2, a sheet support base 3, a presser spring 4, a separating member 5, a final sheet separating plate 6, a sheet detecting arm 7 and a photo-interruptor 8.
In the state shown in FIG. 14(b) wherein the sheets are stocked, a group of the printing sheets stacked on the sheet support base 3 in the sheet delivery cassette 2 are pressed onto the outer circumference of the feed roller 1 under the elastic bias of the presser spring 4. Also, the sheet detecting arm 7 is brought into contact at a free end thereof with the uppermost printing sheet on the sheet support base 3 and lifted up to intercept a light path of the photo-interruptor 8. Thereby, the photo-interruptor 8 detects that the printing sheet is in the sheet delivery cassette 2.
During the sheet feed operation, the feed roller 1 rotates counter-clockwise in the drawing, picks up the uppermost sheet on the sheet support base 3 one by one from the group of printing sheets pressed onto the feed roller 1, and conveys the same in the right direction in the drawing. If two or more printing sheets are picked up together from the upper group of the printing sheets, the separating member 5 disposed downstream from a nip point between the feed roller 1 and the sheet support base 3 is frictionally engaged with the lower side of the plurality of printing sheets picked up together to brake the conveyance of the lower side sheet. As a result, the printing sheets are conveyed one by one. In this regard, it has also been known that a retard roller rotating opposite to the conveying direction may be provided downstream from the feed roller 1.
In the above arrangement, a frictional feeding mechanism and a frictional separation mechanism, each having a rubber surface (or a frictional surface), are generally used in the feed roller 1 and the separating member 5, respectively. In this case, for example, the friction coefficient between the printing sheets (or transfer media) is in a range from 0.4 to 0.7, the friction coefficient between the surface of the separating member 5 and the printing sheet is selected in a range from 0.9 to 1.1, and that between the outer circumference of the feed roller 1 and the printing sheet is 1.5 or more.
FIG. 15 schematically illustrates one prior art laser printer in which the above-mentioned sheet feed device is provided in the sheet delivery part thereof. The operation of this laser printer will be briefly explained below.
During the printing operation, the intensity of a laser beam generated from a laser scanner A is modulated based on image signals fed from a host computer (not shown), and an electrostatic latent image is formed on a photosensitive drum B. On the other hand, the printing sheet in the sheet delivery cassette 2 is picked up one by one by the feed roller 1 as described above, and then conveyed by a conveyor roller C toward the photosensitive drum B while the write timing is adjusted by a register roller D. Then, a toner image on the photosensitive drum B is transferred to the printing sheet by a transfer roller E. Thereafter, the printing sheet passes a conveyor belt F and a fixing roller G to fix the toner image on the printing sheet as a permanent fixed image, and is finally discharged via a discharge roller H and stacked on a tray I.
The above-mentioned prior art sheet feed device has the following problems.
First, when a thick sheet having a basis weight of about 200 g/m2 is fed, a large delivery force is necessary which in turn requires a large capacity drive motor for the feed roller, as well as the friction to the separating member becomes excessive to be apt to generate noise called “squeak”. Also, after the feeding force has been determined to properly feed such a thick sheet having a basis weight of about 200 g/m2, if one wishes to feed a thin sheet having a basis weight of about 60 g/m2, a plurality of thin sheets may be simultaneously fed together to generate a phenomenon called “multi-feed”, which are difficult to be separated even by the separating member.
In addition, since the rubber-like material forming the surface of the feed roller or the separating member is softened or hardened due to the environmental fluctuation, or worn while being used for a long period, there may be a risk in that the friction coefficient thereof varies to deteriorate the sheet feeding performance. Also, it is necessary to use a drive source for the feed roller designed to have a sufficient margin relative to a necessary torque by taking the deterioration with time or the application to a thicker sheet into account.